Francisella tularensis is a gram-negative bacterium recently identified as one of the top-priority agents ("Class A") most likely to pose a potential risk to national security, Although much has been learned from a mouse model using an attenuated strain of Francisella, Francisella tularensis live vaccine strain (LVS), little is currently known about the human immune response to this bacterium. The overall goal of this proposal is gain a better understanding of how different cell populations of the human innate immune system respond to Francisella tularensis. We propose the following hypotheses. LVS and virulent Francisella induce different innate immune responses. Bacterial components other than LPS are important for activating cells of the innate immune response. Three specific aims are proposed to test these hypotheses: 1) Investigate the in vitro response(s) of human peripheral blood mononuclear cells (PBMC) following exposure to F. tularensis versus LVS, 2) Investigate human monocytes and dendritic cells (DC) for their responsiveness to virulent F. tularensis versus LVS, and 3) Investigate human TCRgd T cell and NK cell responses to virulent F. tularensis versus LVS. These specific aims will: a.) Identify the cytokines and cytokine-producing cells present in PBMC cultures following exposure to virulent F. tularensis versus LVS, b) Identify the cell populations that are expanded/activated following in vitro exposure to virulent F. tularensis versus LVS, c) Investigate whether human monocytes and/or DC respond differently to virulent F. tularensis versus LVS, d) Investigate whether is there a difference between virulent Francisella versus LVS with regard to their susceptibility to monocyte versus DC killing, e) Identify the bacterial components that can activate PBMC, monocytes, DCs, NK cells, and/or TCRgd T cells, and f) Investigate whether NK cells and/or TCRgd T cells kill virulent F. tularensis versus LVS. A full understanding of how the innate immune response reacts to Francisella will be extremely important for not only developing a successful vaccine for this pathogen, but also for furthering our understanding of how this bacterium is recognized by the human immune response.